The tabs are already filed off. Just use a liiiiiittle bit of extra force. It’s a very tight fit, but sanwa snap ins will fit into the panel without further modding. The reason why that last 20% isn’t going through is because there are small little nibs under the rim of the bezels that are making it a tad bit harder than the majority of the button to get in.
By shoving it hard, you basically shave off those tabs using the metal of the panel, but since you have art on top of your stick already, it’s probably a lot harder than normal. If you’re handy with an exacto/hobby knife or razor, you can shave off those mini nib-tabs so you don’t have to struggle too hard yourself when actually installing them.
Those nibs are meant to be sized down because they exist for the sole purpose of keeping a tight fit on panels that are a hair bigger than the body of the buttons so that it doesn’t jiggle.
I hope that clears things up and helps you with your installation.